The present invention relates to a monitoring system for detecting a longitudinal (lengthwise) rip in a conveyor belt before the rip reaches a size sufficient to cause excessive or irreparable damage to the conveyor belt or to the conveyor drive system. In large endless conveyor belt systems that are used for transporting bulk materials such as coal, mineral rock and ores over long distances, it is not uncommon for sharp or jagged pieces of the conveyed material as they are dropped onto the conveyor belt to penetrate into the conveyor belt. With continued use of the conveyor belt, the damaged area often can develop into a rip that progressively increases in length longitudinally along the belt until the rip has grown in size sufficient to require the replacement of a large segment of the belt or, sometimes, the replacement of the entire belt. Additionally, if the rip becomes too extensive, the conveyor belt itself may pull apart and become jammed in the drive mechanism of the conveyor system. Accordingly, it has long been recognized that a monitoring system for early detection of longitudinal rips in conveyor belts is desirable.
A number of such monitoring systems have been proposed, but generally the systems are expensive and quite complex in their circuitry thereby increasing the number of potential sources for component failure. Examples of three monitoring systems which previously have been proposed are described in U.S. Pat. Nos. 3,651,506; 3,656,137 and 3,792,459.
The monitoring system described in U.S. Pat. No. 3,651,506 utilizes closed loops of conductive wire embedded in the belt at spaced intervals along the length of the conveyor belt. At selected locations along the path of advance of the belt are positioned a frequency transmitter and a receiver, the transmitter being positioned at one side of the belt with the receiver positioned at the other side of the belt opposite the transmitter. As long as the loops of wire embedded in the belt are not broken, a signal emitted by the transmitter is electromagnetically transmitted by the closed wire loop to the receiver which emits an output signal to an analyzing circuit. The regularly timed pulsing signals from the receiver to the analyzing circuit prevents the triggering of the alarm circuit and the shut-down of the conveyor system. However, if a longitudinal rip develops in the conveyor belt and one of the wire loops becomes broken, no output signal is emitted from the receiver as the broken loop passes between the transmitter and receiver. The interruption to the regular pattern of pulsing signals from the receiver to the analyzing circuit causes the triggering of the alarm circuit and the shut-down of the belt drive mechanism.
U.S. Pat. No. 3,792,459 describes a monitoring system which includes a number of single wire conductors embedded in the belt at spaced intervals along the length of the belt. The conductors extend transversely across substantially the entire width of the belt. As the belt advances the conductors pass over a signal transmitter plate positioned beneath one edge of the belt and a detector plate positioned beneath the opposite edge of the belt. When an unbroken conductor passes over the signal transmitter plate an electrical signal is capacitively induced in the conductor which caues a signal to be capacitively induced at the detector plate. However, when the conductor is broken (for example, by the occurrence of a longitudinal rip in the belt), there will not be a signal inductively produced at the detector plate and an interruption to the regular pulsating pattern of signals emitted by the detector plate will occur causing the triggering of the alarm circuitry and a shut-down of the system.
In U.S. Pat. No. 3,656,137, a monitoring system is described that utilizes a series of spaced conductive wire loops which extend transversely across substantially the entire width of th conveyor belt. Each wire loop is in electrical connection with a turned circuit consisting of a capacitor in parallel electrical connection with an inductance coil. The wire loop serves to "short circuit" the tuned circuit with which it is associated as long as the wire loop remains unbroken. A stationary tuned circuit monitor unit is positioned at the side of the conveyor belt. As the belt is advanced, the wire loops embedded in the belt are caused to pass by the stationary tuned circuit monitor unit. As long as the wire loops remain unbroken, their passage by the monitor unit causes an insufficient effect on the monitoring unit to trigger the alarm circuitry. However, if a rip develops in the belt and breaks a wire loop, the tuned circuit formed by the inductance coil and capacitor with which the wire loop is associated no longer is "short circuited" and as it passes the monitor unit electromagnetically couples with the tuned circuit of the monitor unit resulting in a reduction in the amplitude of the output of the monitor unit which, in turn, triggers the alarm circuit. Although the monitoring system described in U.S. Pat. No. 3,656,137 is not subject to many of the deficiencies inherent in other systems preciously proposed, the use of a capacitor in the wire loop circuits embedded in the belt creates a source of possible malfunction in the system since the capacitors are likely to be damaged and rendered non-functional by the dumping of bulk material onto the belt.